Technical Field
The invention relates to a method for safely switching back to a first computer center following failover by a second computer center and a network structure operating accordingly. The invention relates specifically to a method for failover and to safely switching back a client or a cluster of clients that are configured as transaction terminals, specifically as self-service terminals (e.g. as cash dispensers/ATMs) account statement printers or money transfer terminals, and that are preferably connected to the first computer center and are connected to the second computer terminal only for failover, when synchronization data are exchanged between the two computer centers.
Particularly in the field of transaction terminals and networks that are designed for financial transactions, the transaction terminals can usually be connected to more than one computer center or data center in order to have sufficient backup in the event that the preferred computer center fails. At least one of the additional computer centers then serves as the backup computer center and assumes the tasks of the preferred computer center for the period of downtime. It is known that configuration and transaction data from the connected terminals must be synchronized between the two computer centers. However, if the two computer centers or data centers are at a very great distance from one another, problems can arise with synchronization since the synchronization processes are generally very time-critical. This affects high-availability clusters in particular (known as HA clusters for short; refer also to http:/en.wikipedia.org/wiki/Computer cluster), on which services run continuously on all nodes and thus genuine active/active operation is possible.
Known database management systems also comprise appropriate synchronization mechanisms, such as Oracle RAC (Oracle Real Application Cluster; Oracle® is a registered trade mark of the Oracle Corporation, Redwood Shores, Canada). The Oracle RAC described at http:/en.wikipedia.org/wiki/Oracle. RAC enables failover when several nodes of a cluster access the same database and makes database services available for client computers. If one of the nodes fails, the others assume its functionality. However, it has been shown that these synchronization mechanisms only operate reliably when the databases concerned are not a too great a distance from each other, i.e. are not more than about 25 km from each other. In many network architectures, computer centers and their databases are networked that lie considerably further apart from each other so that on occasion distances of several thousand kilometers have to be overcome. The employment of conventional mechanisms cannot prove successful, particularly in active/active operation; reliable synchronization then requires very great expense with respect to the hardware to be used (including special fiber optic cables, such as dark fiber for example) or it appears to be quite impossible.
Network architectures of the type referred to at the beginning in which preferably transaction terminals, specifically cash dispensers (ATMs; automated teller machines) are connected to computer centers are described in US 2002/0147702 A1 or DE 600 33 895 T2 for example. The problem of safely switching back to the preferred computer center following failover by a far distant backup computer center is, however, not addressed.